The catalyst components of the technology are particularly suited for use in gas-phase processes for the polymerization of olefins, in particular propylene.
The advantage of using gas-phase reactor is well known in the art. When correctly operated this kind of polymerization technology is able to produce polymers endowed with valuable properties with a relatively low investment cost.
Reactor throughput is generally pushed to its maximum by increasing gas mass flow rate up to the value allowed by limit fluidization gas velocity. Exceeding this limit, a significant portion of polymer particles is entrained by recirculation gas: as a consequence, gas recirculation pipe and fan sheeting occurs, heat exchangers tubes and distribution grid plug. As a consequence, the maintenance cost becomes higher, the manufacturing time longer and production losses are also involved.
The entrainment velocity is a direct function of particle size and density. Bigger and/or denser particles allow higher fluidization gas velocity and therefore, in order to optimize the gas velocity, polymer density should be kept up to the maximum value allowed by final application grade. In this connection, the presence of small polymeric fractions, so called fines, which may be generated by irregular catalyst fragmentation during the initial stages of polymerization, is to be avoided as it can cause fouling phenomena such as sheeting of the reactor and of auxiliary apparatuses which in certain cases can even force to stop the polymerization plant.
In this connection, the advantage of using pre-polymerized catalysts is twofold; it makes the catalyst particles bigger and also increases their resistance in such a way that the tendency to break under polymerization conditions is decreased. As a consequence, the catalyst is able to produce bigger polymer particles and also the formation of fines is reduced.
For the polymerization plant not equipped with a prepolymerization section directly connected to the polymerization section, it is possible to feed the polymerization reactor with pre-polymerized catalyst coming from a separate batch prepolymerization unit which provides prepolymerized catalyst with amount of prepolymer per catalyst unit much lower than that obtained in the on-line prepolymerization. WO99/48929 exemplifies preparation of a series of catalysts for use in propylene polymerization prepolymerized to an extent of 1 gr. of polyethylene per gr of catalyst.
Usually, the batch prepolymerized catalyst are stored in drums in an oily slurry form. When needed, the slurry is discharged into a recipient where, after an optional dilution with hydrocarbon medium, undergoes to proper homogenization under stirring.
It has been observed that while homogenizing under stirring very small size polymer material (also called Angel Hair) is produced having high tendency to stick to the disperser wall and to the stirrer baffle. Presumably, such polymer material is released by the pre-polymerized polymer particles which are subjected to shear stress during stirring. Although the amount of polymer released is not very high if considered only a single batch, the continuous mode in which the catalyst is fed, homogenized and discharged leads to a constantly growing deposit of polymer in the disperser that may cause blocking of the stirrer.
It is therefore felt the need of a prepolymerized catalyst generating no or very reduced amount of fine polymer material during the dispersion while at the same time providing the proper behavior in gas-phase polymerization. Now it has been surprisingly found that when a prepolymerized catalyst having the following characteristics is used, the formation of angel hair during the dispersion stage is greatly reduced.